A light emitting diode (LED) is a semiconductor device that is configured to receive electrical power to stimulate an output of electromagnetic radiation commonly in the visible range of the spectrum (light). Portions of a LED comprise doped semiconductor materials that operate to combine charge in a way that releases light energy from the body of the LED material. While the irradiance depends on the operating conditions of the LED, the wavelength of the light energy is determined by the band gap of the semiconductor materials, in addition to external interaction with various active optical covers and encapsulants.
Packaging of electronic devices, such as light emitting diodes (LEDs) and other devices, represent a major cost in the production of electronic parts. In one non-limiting example, LEDs which offer long lifetime, compact form factor, superior energy efficiency, and RohS compliancy are expensive due to the packaging requirements which include sealing, optics, phosphor and efficient heat conduction. There have been numerous efforts to reduce the cost of the electronic device packaging by using silicon based wafer level assembly technologies. However, these approaches still require a carrier chip for the electronic device and in most cases the carrier chip doubles the cost, and in the case of an LED triples the heat resistivity.
Specifically, when electrical current is passed through a LED (i.e., switched on), carrier electrons recombine with holes within the device releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs are often small in radiative area (less than 1 mm2). Integrated optical components may be used to shape or change its radiation pattern.
A LED package can also incorporate and perform optical functions. For example, a LED package can include optical materials and/or structures, such as lenses, diffusors, light scattering layers, etc., that can direct light output by the semiconductor chip in a desired manner. These are typically bonded to the transmission face of the LED.
Light emitting devices generally include a semiconductor chip, or die, including a p-n junction formed upon an epitaxial layer grown on a substrate, such as, sapphire, silicon, silicon carbide, or gallium arsenide. The substrate may subsequently be trimmed, patterned, or removed altogether. In addition to phosphor luminescent transitions, the wavelength distribution of the light generated by the LED depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device.
The p-n junction semiconductor die is typically enclosed in a package. A LED package can perform a number of functions and provide a number of benefits. For example, a LED package can provide mechanical support and environmental protection for the semiconductor die, as well as providing electrical leads for connecting the die to an external circuit, and heat sinks for efficient heat extraction from the chip.
It is often desirable to incorporate phosphor into a solid state light emitting device package to enhance the emitted radiation in a particular frequency band and/or to convert at least some of the radiation to another frequency band. Depending on the application, phosphorescent coatings and/or suspensions are often used to broaden the bandwidth of the emitted light. Phosphors absorb some of the photons emitted from the LED and temporarily stores its energy before releasing it in another wavelength photon.
In general, phosphors absorb light having shorter wavelengths and re-emit light having longer wavelengths. As such, some or all of the light emitted by the LED chip at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a single blue emitting LED chip may be surrounded with a yellow phosphor, such as cerium-doped yttrium aluminum garnet (YAG). The resulting light, which is a combination of blue light and yellow light, may appear white to an observer. Photo luminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanoseconds. Light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate.
Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. LEDs are used as indicator lamps in many devices and are increasingly used for general lighting. Appearing as practical electronic components in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.
LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. However, LEDs powerful enough for room lighting are relatively expensive, and require more precise current and heat management than compact fluorescent lamp sources of comparable output.
LEDs compare favorably to other sources of light and are especially useful in certain applications and markets. For example, LED lighting generally provides advantages with respect to energy efficiency, compact, rugged, long-lasting design and form factor, as well as other features. LED lighting compares favorably with other sources in the amount of light energy generated in the visible electromagnetic spectrum compared to the infra-red or heat energy wasted by the light source. In addition, LED lights include fewer environmentally damaging components when compared to other light forms, and therefore provide better compliance with restrictions on hazardous substances (RohS) regulations.
That said, conventional LED devices can be relatively costly to manufacture by some metrics when compared to other light sources. One reason for this is the exacting packaging requirements for manufacturing LEDs. LED packaging calls for proper clean conditions, micro-fabrication facilities similar to other semiconductor manufacturing operations, sealing requirements, optical requirements, the use of phosphor in LED applications, as well as packaging that is designed to efficiently handle the conduction of heat generated in the devices.
Efforts have been made to reduce the cost of conventional LED packaging which uses silicon (Si) or ceramic based carrier substrates. LEDs mounted on an individual chip scale carrier substrate are more expensive to process. Alternatively, LEDs are mounted on a carrier wafer, where the packaging process is done on many LEDs in parallel and the packaged LEDs are singulated at the end of the packaging process, results in a lower cost for the packaged LEDs. The wafer based approach is termed wafer level assembly packaging (WLP).
Wafer-level packaging consists of extending the wafer fab processes to include device interconnection and device protection processes. Other types of packaging performs wafer dicing first and places the individual die in a plastic package followed by attaching the solder bumps. Wafer-level packaging involves attaching the top and bottom outer layers of packaging, and the solder bumps, to integrated circuit while still in the wafer, and then wafer dicing.
However, these conventional techniques still require the use of a carrier substrate to support the LED, which can double the cost of making and packaging the LED device. In addition, the carrier substrate greatly increases the thermal resistivity of the device and adversely affects its heat removal characteristics. Hence it is desirable to provide a wafer level package for LEDs which does not require any carrier substrate and uses the LED die only, alternatively, the package provides a direct thermal connection from the LED to the heat sink for efficient removal of the generated heat. Accordingly, there is a need for LED devices that do not suffer from some or all of the above problems.